Yup the afm is a larger mechanical unit with a piece of plastic scewed into it (the potentiometer) this is the thing that breaks. MB only sells the whole unit unfortunately, but you can by the potentiometer seperately now.
m

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Martin Ingram
Colorado Springs
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)